The ease and effects of underpitching are way worse than the inease and effects of overpitching, though.
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jsled♦Jan 21 '11 at 16:17

I down voted this for a few reasons: a yeast package may say how high a gravity it can ferment, but that depends on the yeast being fresh. And "some people" might be right about dry yeast, but a link to details would be improve this answer.
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HopwiseJan 21 '11 at 16:23

I recommend making a yeast starter for every beer you use liquid yeast for (I have little experience with dry yeast because I stopped using it after my first few batches). You don't need a starter all the time, but it's on my list of best practices.

You will almost always see better fermentation if you use a starter. It will start faster (which helps a ton to prevent the bad bugs from getting at your wort before the yeast get into full swing), and you'll be certain that you have healthy yeast to make a beautiful-tasting beer.

Even if you forget to make a starter well before you brew (1-3 days), you will benefit from making a small starter on the morning of your brew day just to ensure you've got some healthy yeast and so your fermentation kicks off quicker.

The Lazy Brewer's Guide to a Simple Starter (assuming you know how to make a starter and you want to be a little lazy or forgot to make one before brew day).

I measure water with my Erlenmeyer Flask at the size of the starter--the boil off + DME volume tends to even it out pretty much perfectly on a little below medium heat on my stove. So, for a 1000 ml starter I use 1000 ml of water. Simple.

Boil for 10 minutes. It should be vigorous enough that the wort turns over, but shouldn't be making liquid jump off the surface.

Cool the wort in your flask or starter vessel with foil on top (sanitizes the foil if you don't have a spray bottle of Star San or equivalent on hand... and keeps bad bugs out).

Shake it up, add your yeast. Shake it up whenever you think about it.

Add it to your cooled wort after brewing.

The number of steps makes it sound like a pain, but I probably spend 5 minutes of effort on any of my starters and it makes my beer better.

Really, you don't ever have to use a starter. However, I've found that if I don't have time to make a good starter, using two vials of yeast is a viable alternative. In fact, after discovering that it works very well, I've gotten noticeably more lazy in making starters in time for brew day. :)

You use a starter primarily to insure you have enough yeast. If you use two vials of yeast, you can forego the starter.

I'll go further to add that, although there's a lot of talk about "must haves", starters being one of the big ones, the only "must haves" in brewing are good ingredients, and good hygiene. Any other time you hear about a 'must have', think about what it accomplishes: chances are good there's another way to do it. Starters are a perfect example.

You should always pitch the appropriate amount of yeast. Whether it comes from a starter or more packages of yeast from the store is secondary to that. So if you pitch 5 vials of yeast that's fine to get there rather than do a 4 L starter, that's fine.

A starter has the advantage of pitching active or healthier yeast than whats in the packages usually.

1.070 is a weird place to make the cut. I think 1.040 is a more appropriate gravity to draw that grey line.

There isn't really a 'hard line' of gravity above which you must use a starter, that is because yeast packages are not always the same age.

Say you're making a beer that has an OG of 1.040 and you buy a yeast vial that is only one week old. 92 % of the yeast in that vial is viable (according to Jamil's calculator). So you could probably go without a starter.

But if that vial is a month old? 75% viable, and now you need a starter. 2 months? 54% viable.

As others have said here the goal is to use the right amount of yeast. But the real goal is to use the right amount of healthy, viable yeast. Using a calculator like Jamil's will help you figure out what that amount is.